1. Field of the Invention
The invention relates to a fill level sensor having an ultrasonic transmitter and receiver for detecting the fill level of a liquid in a container.
2. Description of Prior Art
Fill level sensors of this type are used in very diverse applications and for different liquids. Ultrasonic waves are emitted with the aid of the ultrasonic transmitter and are reflected from a boundary surface of the liquid in contact with an adjacent medium. The reflected ultrasonic waves are received by the ultrasonic receiver. By suitable evaluation a fill level of the liquid in the container can be determined, for example on the basis of the transit time of the sound waves. Deformations of the boundary surface caused by movements of the liquid, or foaming on the surface of the liquid, can adversely affect this measuring method. It is therefore known to insert so-called calming tubes within the liquid, which damp the movements of the boundary surface and counteract foaming.
The aforementioned problems occur, in particular, when measuring the fill level of an oil tank in a motor vehicle, since the oil sloshes constantly back and forth in the tank while driving and in addition has a tendency to foam, accentuated in some cases by vibrations of the vehicle. Fill level sensors have therefore become known for use in such oil tanks in motor vehicles, and a detail of this known arrangement is shown in FIG. 4. The fill sensor has an inner tube 18 for arrangement in the oil tank, which inner tube is arranged above the ultrasonic transmitter and receiver. Oil contained in the oil tank forms a liquid column in the inner tube 18 the height of which is detected using the ultrasonic transmitter and receiver. The inner tube 18 is surrounded by an outer tube 20. A cap 22 covers the inner tube 18 and the outer tube 20 towards the top. The inner tube 18 serves to guide the ultrasound. The upper boundary surface of the liquid column in the inner tube 18 in contact with the air above it is effectively calmed by the two tubes 18, 20 arranged one inside the other. In addition, the cap 22 prevents oil from splashing into the inner tube 18 or into the outer tube 20 from above while driving, which further calms the boundary surface and counteracts foaming.
In the known arrangement, the lower ends of the inner tube 18 and of the outer tube 20 are usefully located close to the lowest point of the container. The upper ends of the inner tube 18 and of the outer tube 20, and the cap 22, are located above the highest fill level to be detected. The lengths of the inner tube 18 and of the outer tube 20 must therefore be adapted exactly to the particular application.